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Speak to enter is a puzzle based on trickery. The puzzle is as follows: The party comes across a living door, which will refuse to open until one specific objective has been completed. Above the door is a text that reads "Speak bird to enter.", bird can be replaced with any word you see fit.
The party will likely think bird is a password, so they'll likely say it, perhaps even in various languages. Still, the door will not open. It'll talk back, but will generally mock them. The trick is to get the door to say the word, not the party. Once the party figures this out it becomes a game of wits, one you can make as challenging or easy as you wish. Perhaps they have to trick you into saying the word, or perhaps they simply have to make a deception roll for it.
The puzzle can obviously be altered in all sorts of ways. Instead of a door it could be a hulking figure blocking a path, a savy trickster wagering some gold, or perhaps a Rumpelstilskin-esque adventure. No matter what, it offers for some fun role playing opportunities.
I do advice you to keep it simple enough for the players to resolve. If you want to see if your players can make you say the word yourself only go along with it for a short while, your players might get frustrated otherwise.